Skip commit and push when playlist has no changes

Prevents the workflow from failing with exit code 1 when the generated
playlist is identical to the previous run and there is nothing to commit.

.github/workflows/update_playlist.yml

@@ -17,5 +17,5 @@ jobs:
           git config user.email "playlistbot@users.noreply.github.com" || true
           python3 ./make_playlist.py
           git add .
-          git commit --quiet -m "Update Playlist (GitHub Actions)"
-          git push -f origin master
+          git diff --staged --quiet || git commit --quiet -m "Update Playlist (GitHub Actions)"
+          git diff --quiet HEAD origin/master || git push -f origin master